142 BRITISH STROMATOPOROIDS. 



confluent into vermiculate rows. There do, however, occur specimens in which the 

 typical " hexactinellid " structure can be detected, in parts at any rate, in tangential 

 slices. In these cases (Plate XIV, fig. 5) the " arms " given out by the radial 

 pillars are numerous, and almost thread-like or capillary in point of size. The 

 general skeletal framework in the present species is decidedly closer and more 

 dense than in A. hebbornense or A. clathratum ; and a very well-marked and character- 

 istic feature in vertical sections (Plate XIV, figs. 4 and 6) is the presence of large 

 rounded apertures caused by the cutting across of the radiating astrorhizal tubes. 

 These apertures are commonly distributed in irregular vertical lines which 

 correspond with the separate astrorhizal systems. Vertical sections, also, often 

 show the vertical wall-less canals from which spring the astrorhiza? of successive 

 interlaminar spaces and round which the concentric lamina? are usually bent 

 upwards (Plate XIV, fig. 2). 



As regards its form and mode of growth A. stellulatum is very variable, but 

 its variations may be reduced to the following three chief types : 



1. The first type includes forms in which the ccenosteum grows as a larger 

 or smaller laminar expansion of comparatively small thickness, and having a 

 concentrically wrinkled basal epitheca. Growth by " latilaruina? " is not marked, 

 and the surface is destitute of " mamelons." The concentric lamina? are nearly 

 plane or only gently wavy, and are never sharply undulated. Lastly, the astrorhiza? 

 are comparatively large, with numerous long, slender, radiating branches, and 

 have their centres from 6 to 8 mm. or moi"e apart. This laminar variety is 

 common at Gees and at other localities near Gerolstein, but it is of doubtful 

 occurrence in Devonshire. 



2. The second type comprises massive forms, often of large size, more or less 

 spheroidal or irregular in shape, and without a basal epitheca. Very commonly 

 the ccenosteum is markedly composed of successive " latilamina?," and the 

 concentric lamina? are often more or less undulated. The surface is sometimes 

 nearly smooth or gently wavy, but it is very generally covered with low, rounded 

 " mamelons," which average 5 — 6 mm. apart, and mostly correspond with the 

 axial canals of the astrorhiza?. The astrorhiza? are mostly of small size with few 

 radiating branches, and these comparatively short. The forms of this group are 

 extremely abundant at Gerolstein in the Eifel, and they are not very rare in 

 Devonshire. I have figured (Plate XV) a beautiful pyriform specimen from 

 Devonshire which Mr. Vicary was so kind as to place in my hands for 

 examination. 



3. A third well-marked group of forms comprises examples in which the 

 ccenosteum appears in transverse sections (Plate XIV, fig. 1) as if made up of 

 parallel cylinders, the interspaces between which are filled up by undulated 

 lamina?. The cylinders vary from 1 — G cm. in diameter, and longitudinal sections 



